Attempting to cure LieutenantReginald Barclay of his Urodelan flu, Dr. Crusher used synthetic T-cells to activate the gene that would fight off the infection. However, an anomaly in Barclay's genetic structure caused the T-cell to mutate and activate all of Barclay's dormant genes, including his introns. The T-cells then became airborne, spreading throughout the ship like a virus and causing the crew to "de-evolve." The syndrome was cured by using amniotic fluid from Alyssa Ogawa to create a retrovirus, eliminating the effects.

Initial symptoms of the disease included abnormal behavior, irritability, hyperactivity or decreased activity, chills, fever, and paranoia. Later, the newly activated introns transformed the crewmembers into many different earlier lifeforms from their respective homeworlds. (TNG: "Genesis")